1. Field of the Invention
The present invention relates to a method of removing a scale caused by a raffinate. More particularly, it relates to a method of removing the scale deposited from a raffinate in a purification of a phosphoric acid prepared by a wet process with an organic solvent or a mixture of an organic solvent and a strong acid by an extraction method which comprises contacting the scale with a solution containing a strong base.
2. Description of the Prior Art
It has been known that the phosphoric acid prepared by decomposing a phosphate rock with a mineral acid such as sulfuric acid (hereinafter referring to as a wet process phosphoric acid) is purified by an extraction method with an organic solvent which is miscible to phosphoric acid but has low mutual solubility to water such as various alcohols, ketones, ethers and phosphoric acid esters.
The wet process phosphoric acid includes various impurities derived from the phosphate rock such as iron and aluminum compounds.
In the extraction method, the impurities are precipitated as a mixture containing main components of iron and aluminum phosphates.
The precipitate is deposited as a scale to clog an extracting apparatus especially an outlet of the raffinate, a solvent recovering device for recovering a solvent from the raffinate and fittings such as pipes for connecting the devices.
It has been known to prevent the deposition of the scale by a method of using an organic acid containing a strong acid such as sulfuric acid, hydrochloric acid or nitric acid. However, it is difficult to prevent the deposition of the scale when the concentration of P.sub.2 O.sub.5 in the raffinate is lower than certain limit.
The inventors have studied the formation of the scale and have found that the composition and the amount of the scale has close relation to the concentration of P.sub.2 O.sub.5 in the raffinate as follows.
When the concentration of P.sub.2 O.sub.5 in the raffinate is higher than 20 wt.%, the scale is not substantially deposited, whereas when it is less than 20 wt.%, the scale is deposited.
The composition of the scale deposited in 5 to 20 wt.% of the concentration of P.sub.2 O.sub.5 in the raffinate, contains main components of iron and aluminum phosphates. The deposition of the scale can be prevented by using a mixture of an organic solvent and a strong acid as a solvent for extraction.
When the concentration of P.sub.2 O.sub.5 in the raffinate is lower than 5 wt.%, the amount of the scale deposited, is remarkably increased and the composition of the scale is a mixture containing main components of titanium and zirconium phosphates which is different from the former scale and is substantially insoluble in a strong acid to firmly deposite.
In the method of preventing the deposition and accumulation of the scale from the raffinate by contacting a mixture of an organic solvent and a strong acid with the wet process phosphoric acid, the composition and the properties of the scale are different depending upon the concentration of P.sub.2 O.sub.5 in the raffinate. Accordingly, the deposition and accumulation of the scale are not always prevented by the method of treating with a strong acid.
The inventors have found the above-mentioned facts and considered to overcome the technical problem and have studied various methods of removing the scale which is firmly deposited.